The use of blocking agents for temporary protection of isocyanate groups has been known for a long time. Blocked polyisocyanates are employed, inter alia, for the preparation of thermosetting 1C PU stoving systems which are stable to storage at room temperature. The blocked polyisocyanates are mixed here e.g. with polyesters containing hydroxyl groups, polyacrylates, other polymers and further constituents of lacquers and paints, such as pigments, co-solvents or additives. Self-crosslinking one-component stoving systems which contain, as binders, polymers which contain both blocked isocyanates and hydroxyl groups in one molecule are another form of stoving lacquers which are stable to storage at room temperature.
Overviews of the use of blocked polyisocyanates are to be found, for example, in Wicks, Z. Progress in Organic Coatings 3 (1975) 73–99, Wicks, Z. Progress in Organic Coatings 9 (1981) 3–28, D. A. Wicks and Z. W. Wicks, Progress in Organic Coatings, (1999), 148–172.
The most important compounds which are employed for blocking polyisocyanates are ε-caprolactam, methyl ethyl ketoxime (butanone oxime), malonic acid diethyl ester, secondary amines and triazole and pyrazole derivatives, such as are described e.g. in EP-A 0 576 952, EP-A 0 566 953, EP-A 0 159 117, U.S. Pat. No. 4,482,721, WO 97/12924 or EP-A 0 744 423.
Secondary amines are described as blocking agents in EP-A 0 096 210. However, only amines containing alkyl, cycloalkyl and aralkyl groups are mentioned expressly as blocking agents there. Amines which contain functional groups with carbon-heteroatom multiple bonds or heteroatom-heteroatom multiple bonds are not mentioned explicitly there.
The most frequently employed blocking agents for isocyanates are ε-caprolactam and butanone oxime. While as a rule stoving temperatures of about 160° C. are used in the case of ε-caprolactam, blocked 1C stoving lacquers in which butanone oxime has been employed as the blocking agent can already be stoved at temperatures 10 to 20° C. lower. Nevertheless, at these stoving temperatures the desired lacquer properties are no longer achieved in some lacquer systems. These temperatures, however, are meanwhile found to be too high, so that there is a need for stoving systems which crosslink completely at lower temperatures than when systems containing butanone oxime-blocked isocyanate crosslinking agents are employed.
The present invention was therefore based on the object of providing blocked polyisocyanates which have a lower crosslinking or stoving temperature than butanone oxime-blocked polyisocyanates.